NASHVILLE (AP) — Four environmental organizations on Wednesday asked the Tennessee Valley Authority's internal watchdog to investigate whether the nation's largest public utility misused ratepayer money for lobbying and litigation that fought federal environmental regulations.

The request to the TVA's Office of Inspector General comes after the Energy and Policy Institute, an activist group, released more than 500 pages of records it obtained through a Freedom of Information Act request. The documents prove TVA's dues to the now-disbanded Utility Air Regulatory Group were used for unauthorized activities, the group said. They point to expense reports showing most of UARG's money went to a law firm. In lawsuits, the UARG frequently argued against tighter air pollution and climate regulations.

TVA CEO Jeff Lyash told Congress in a 2019 letter that the utility had contributed $7.3 million to the Utility Air Regulatory Group since 2001. Lyash said the funds were not used to lobby or sue on behalf of TVA, which would require explicit board approval. Instead, TVA used its membership in the regulatory group "to help understand, plan for, and comply with highly technical and complex regulations developed" under the Clean Air Act, Lyash wrote.

The environmental groups seeking a review of the relationship point to documents from the UARG's Nonattainment Committee, which was co-chaired by Don Houston, a TVA senior manager. In that role, he approved billing hours and invoices from Hunton Andrews Kurth, a Washington, D.C., law firm that worked for the group, documents show.

Expense reports for the years 2015 through July 2018 show the committee spent nearly $3.5 million on legal fees and expenses and only about $48,000 on technical expenses. While TVA may not have put its name on lobbying efforts and lawsuits, it was paying for them, the environmental groups allege.

TVA did not directly respond to the new allegations but said in a written statement that "contacting the OIG or any other TVA oversight groups is an appropriate avenue for any member of the public to raise potential concerns."

The statement added: "Although TVA supported the decision to disband the Utility Air Regulatory Group (UARG) in May 2019, our customers have directly benefited from the research and technical expertise gained from UARG participation, including significantly reduced air emissions and cleaner energy."

A spokesperson for the inspector general's office, Terri Beatty, declined to comment on the letter, which also calls for a review of any TVA involvement in other trade groups, including the Utility Solid Waste Activities Group, the Utility Water Act Group, the Clean Air Act Monitoring Service and the Climate Legal Group.

TVA should not be "suing the federal government over existing laws and regulations, especially when they are part of the federal government," said Daniel Tait, chief operating officer of Energy Alabama, one of the nonprofits requesting the review. "Even if they were a private utility, these are bedrock environmental regulations that protect public health."

The environmental groups — which also include the Center for Biological Diversity, Appalachian Voices and Southern Alliance for Clean Energy — sent a similar request to the inspector general's office in 2019. Tait said they do not know whether the watchdog agency has acted on their previous request, but with the new request they have more evidence of problems they merely suspected two years ago.

The Tennessee Valley Authority provides power to nearly 10 million people in parts of seven Southern states and is the country's third largest electricity generator.